Method of preparing a product based on phosphate of thorium and/or actinide(s)

ABSTRACT

The invention relates to a process for the preparation of a product based on a phosphate of at least one element M(IV), for example of thorium and/or of actinide(IV)(s). 
     This process comprises the following stages:
         a) mixing a solution of thorium(IV) and/or of at least one actinide(IV) with a phosphoric acid solution in amounts such that the molar ratio       

     
       
         
           
             
               PO 
               4 
             
             
               M 
               ⁢ 
               
                   
               
               ⁢ 
               
                 ( 
                 IV 
                 ) 
               
             
           
         
       
         
         
           
              is from 1.4 to 2, 
             b) heating the mixture of the solutions in a closed container at a temperature of 50 to 250° C. in order to precipitate a product comprising a phosphate of at least one element M chosen from thorium(IV) and actinide(IV)s having a P/M molar ratio of 1.5, and 
             c) separating the precipitated product from the solution. 
           
         
       
    
     The precipitate can be converted to phosphate/diphosphate of thorium and of actinide(s). The process also applies to the separation of uranyl ions from other cations.

TECHNICAL FIELD

A subject-matter of the present invention is the production of productsbased on phosphate of thorium and/or of actinides which can be used forthe conditioning and treatment of reactive waste, such as liquideffluents.

More specifically, it relates to the preparation of precursor productsof thorium phosphate/diphosphate of formula Th₄(PO₄)₄P₂O₇ (TPD) or offormula Th_(4-x)M_(x)(PO₄)₄P₂O₇ in which M is a tetravalent actinide,such as Pa, U, Np and Pu.

STATE OF THE PRIOR ART

The document WO 96/30300 [1] disclosed several processes for thesynthesis and characterization of thorium phosphate Th₄(PO₄)₄P₂O₇ asconditioning matrix for the storage of nuclear waste.

The syntheses relate to various media. For the syntheses which takeplace in a liquid medium, they report a mixture of aqueous solutions ofchemicals (thorium salts, compounds comprising nuclear waste, acids,bases) which always results in concentrated solutions.

A dry and amorphous residue is obtained from these concentratedsolutions, mainly by evaporation of the volatile materials (water andacids, for example), which residue will result, by calcination at850–1300° C., in the fully crystalline thorium phosphate/diphosphateTh₄(PO₄)₄P₂O₇, which then has the properties required for good retentionof the nuclear waste which is incorporated therein.

In this process, the stage which consists in carrying out theevaporation of concentrated solutions is entirely achievable but underextreme conditions. This is because it concerns the evaporation ofchemically highly aggressive solutions (acidic or basic) comprising veryhigh levels of radioactivity. This route is therefore technicallypossible but difficult.

ACCOUNT OF THE INVENTION

A subject-matter of the present invention is specifically a process forthe preparation of a product based on phosphate of thorium and/or ofactinide(s) which can be easily converted to thoriumphosphate/diphosphate without requiring a stage of evaporation ofaggressive solutions.

According to the invention, the process for the preparation of a productcomprising a phosphate of at least one element M(IV) chosen fromthorium(IV) and actinide(IV)s is characterized in that it comprises thefollowing stages:

-   a) mixing a solution comprising thorium(IV) and/or at least one    actinide(IV) with a phosphoric acid solution in amounts such that    the molar ratio

$\frac{{PO}_{4}}{M\;({IV})}$where M(IV) represents the total concentration of thorium(IV) and/oractinide(IV)s, is from 1.4 to 2, preferably from 1.5 to 1.8,

-   b) heating the mixture of the solutions in a closed container at a    temperature of 50 to 250° C. to precipitate a product comprising a    phosphate of at least one element M chosen from thorium(IV) and    actinide(IV)s having a P/M molar ratio of 1.5, and-   c) separating the precipitated product from the solution.

Thus, according to this process, the starting materials are exclusivelyacidic solutions having a substantially identical chemical compositionto that described above, and a crystalline precursor is obtained by mildchemistry, that is to say by moderate heating of the mixture in a closedcontainer, to avoid, this time, the evaporation and to promote theprecipitation of this precursor. The latter will subsequently be easilyseparated, by settling, filtration or centrifuging at ambienttemperature, from the solution which has given rise to it. This processthus avoids the stage of evaporation and the treatment of the gaseouseffluents inherent in the process disclosed in WO 96/30300 [1].

This is particularly advantageous when the process is used for thetreatment of liquid effluents as the need to evaporate radioactiveacidic solutions and to treat the gaseous effluents resulting from theevaporation is thus avoided.

According to the invention, the heating is carried out in a closedchamber at a moderate temperature (50 to 250° C.) for a variable periodof time which depends on the temperature used and on the nature ofM(IV). This period of time can range, for example, from 1 hour to 1month, in order to obtain a finely crystalline solid which can be easilyseparated from the solution after the latter has been cooled.

The heating time is an important parameter as it has a direct influenceon the quality of the precipitate formed.

For this heating, the mixture of the solutions of M(IV) and ofphosphoric acid is introduced, for example, into a Teflon container witha screw cap which is highly leaktight.

The container can be heated by any means, for example in an electricresistance oven, in a microwave oven, in a sand bath, in an oil bath oralternatively by using an infrared lamp or a stream of hot gas.

According to one advantageous characteristic, the process for theinvention furthermore comprises the following stages:

-   d) washing with water the precipitated product thus separated, and-   e) drying the washed product, for example in an oven.

The solutions comprising thorium and/or one or more actinides which canbe used in this process can be prepared, for example, from salts, suchas chlorides, bromides, nitrates, sulphates and oxalates.

It is also possible to prepare them by any method which makes possiblethe introduction of actinide(IV)s ions into solution, for example bydissolution of the metal or of the oxide.

The process described above can be used to prepare thoriumphosphate/diphosphate of formula Th₄(PO₄)₄P₂O₇ by using, in stage a), asolution comprising thorium and phosphoric acid and by subjecting theproduct based on thorium(IV) phosphate, separated by precipitation, to aheat treatment carried out at least partially at a temperature of 700 to1300° C.

It is also possible to use the process of the invention to prepare asolid solution of phosphates of thorium and of at least one tetravalentactinide by using, in stage a), a solution comprising thorium and atleast one tetravalent actinide and phosphoric acid and by subjecting theproduct based on phosphates of thorium(IV) and of actinide(IV) (s)obtained to a heat treatment carried out at least partially at atemperature of 700 to 1300° C.

For example, the heat treatment is carried out in two stages, a firstprecalcination stage carried out at a temperature of 300 to 500° C. for1 to 5 h and a second calcination stage carried out at a temperature of1100 to 1300° C. for 3 to 15 h.

It is also possible to carry out a cold compacting of the powder beforecarrying out the heat treatment, in order to obtain a sintered product.

Thus, the process of the invention makes it possible to obtain asintered product under better conditions (temperature, duration,pressure) than in the case of the document [1], because of the betterphysicochemical properties (particle size, specific surface) of theprecipitated product.

It is thus possible to prepare solid solutions of phosphatescorresponding to the formula:Th_(4-x)M_(x)(PO₄)₄P₂O₇in which M is an element chosen from Pa(IV), U(IV), Np(IV) and Pu(IV),and x satisfies the following conditions:

-   -   x≦3.75 for Pa(IV)    -   x≦3 for U(IV)    -   x≦2.14 for Np(IV)    -   x≦1.67 for Pu(IV).

For the implementation of the process of the invention, it is possibleto use, for example, as thorium solution, a solution of ThCl₄ inhydrochloric acid or a solution of thorium nitrate in nitric acid. TheThCl₄ solution can be obtained by dissolution of solid ThCl₄ in a 0.5 to2M hydrochloric acid solution, in order to obtain a 0.5 to 2M ThCl₄solution. The solution of thorium in a nitric acid medium can beobtained by dissolution of Th(NO₃)₄.5H₂O in a 0.5 to 5M HNO₃ solution,in order to obtain a 0.5 to 2M Th solution.

When the actinide is uranium(IV), the uranium(IV) solution can be asolution of UCl₄ in hydrochloric acid, for example obtained bydissolution of UCl₄ in 0.5 to 6M HCl or by dissolution of uranium metalin 6M HCl, subsequently brought to the desired concentration, forexample 0.5 to 1.5M in U, by dilution with deionized water.

When the actinide is neptunium(IV), the neptunium(IV) solution can be asolution of neptunium in nitric acid, for example obtained bydissolution of solid NpO₂ in 4 to 5M HNO₃ and dilution with deionizedwater, in order to have a nitric acid concentration of 1 to 4M and an Npconcentration of 0.1 to 0.3M.

When the actinide is plutonium(IV), the plutonium solution can be asolution of plutonium in nitric acid, for example obtained bydissolution of PuO₂ in a 4 to 5M nitric acid HNO₃ solution and dilutionwith deionized water, in order to obtain a nitric acid concentration of1 to 4M and a Pu concentration of 0.2 to 0.6M.

Thus, the process of the invention makes it possible to prepare thoriumphosphate/diphosphate when it is carried out with a thorium solution towhich a phosphoric acid solution is added in the desired molar ratio. Agel is thus formed, which gel, in the closed container, is subsequentlyconverted by heating to a precipitate formed of a crystalline powder.This powder can subsequently be converted to thoriumphosphate/diphosphate (TPD) by heat treatment.

The process of the invention can also be used to prepare an actinidephosphate, for example a uranium(IV) phosphate, by first of all forminga uranium phosphate precipitate which is subsequently converted by heattreatment to a phosphate with a different structure from that of thoriumphosphate/diphosphate, corresponding to a polyphase system.

For this preparation, use is made, in stage a), of a solution comprisinguranium and phosphoric acid, and the precipitated product is subjectedto a heat treatment carried out at least partially at a temperature of700 to 1300° C.

In this case, the acidic uranium solution must not comprise an oxidizingagent as, in the presence of an oxidizing agent, such as, for example,nitric acid, the tetravalent uranium is oxidized to hexavalent uraniumin the form of the uranyl ion UO₂ ²⁺, which will not form a precipitateunder the conditions of the process of the invention.

The process can also be used to form solid solutions of tetravalentactinides starting from a mixture of acidic solutions comprising severalactinide(IV)s and of phosphoric acid, which is subsequently subjected toheating in a closed container at a temperature of 50 to 250° C. toprecipitate a product comprising actinide phosphates.

When the starting material is a solution of thorium and of actinide(s),a very homogeneous powder composed of particles with a size of less than3 μm is obtained by precipitation followed by heating in the closedchamber, which powder can be converted by heat treatment to aphosphate/diphosphate of thorium and of uranium(IV).

It is also possible to include trivalent elements or other tetravalentelements in this phosphate/diphosphate of thorium and/or of tetravalentactinide(s) by coprecipitating the trivalent elements with thetetravalent elements during the heating in a closed chamber. At the endof the operation, a two-phase or polyphase system composed ofphosphate/diphosphate of thorium and/or of tetravalent actinide(s), andof a phosphate comprising trivalent lanthanide elements, such asgadolinium and lanthanum, and/or trivalent actinide elements, such asamericium and curium, is obtained.

It is also possible to prepare, from the product based on phosphate ofthorium(IV) and/or of actinide(IV) (s) obtained by precipitationfollowing stages a) to c), a composite material including at least oneactinide(III) and/or at least one lanthanide(III) in the phosphate form,such as monazite M(III)PO₄, xenotime M(III)PO₄ and brabantiteM(II)_(x)M′ (IV)_(x)M″ (III)_((2-2x)) (PO₄)₂.

This material can be prepared by dispersing a powder of the phosphate(s)formed beforehand in the precipitate of the product comprising aphosphate of thorium(IV) and/or of actinide(IV)(s) and by subsequentlysubjecting the combination to a heat treatment, optionally preceded by acompacting, carried out at least partially at a temperature of 700 to1300° C.

A further subject-matter of the invention is a process for theseparation of uranium(VI), in the form of the uranyl ion UO₂ ²⁺, presentin a solution with other cations, including thorium, according to whichphosphoric acid is added to the solution in an amount such that thephosphoric acid/other cation molar ratio is from 1.4 to 2, the solutionthus obtained is heated at a temperature of 50 to 250° C. in a closedcontainer, in order to precipitate a product comprising the cationsother than uranium, and the solution comprising uranium(VI) isrecovered.

This is because uranyl phosphate (UO₂)₃(PO₄)₂·5H₂O, which is moresoluble than phosphates of trivalent and tetravalent cations, such as,for example, actinides and lanthanides, may not precipitate undercertain operating conditions. It is therefore possible to separateuranium from the other cations by varying the concentrations of theentities in solution and the acidity of the medium, so as to form aprecipitate comprising the other cations and to leave the uranium insolution.

A further subject-matter of the invention is a process for thedecontamination of radioactive aqueous effluents which consist inprecipitating a product based on thorium phosphate from the effluent byaddition of thorium and then of phosphoric acid to the effluent inamounts such that the P/Th molar ratio is from 1.4 to 2 and in heatingin a closed container at a temperature of 50 to 250° C. in order toprecipitate a product comprising thorium phosphate from the effluent,thus entraining the contaminating radioactive cations.

Other characteristics and advantages of the invention will become moreclearly apparent on reading the description which follows of examples,of course given by way of illustration and without implied limitation,with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an X-ray diffraction diagram of the thoriumphosphate/diphosphate precursor obtained in Example 1;

FIG. 2 is the infrared spectrum of this same precursor;

FIG. 3 is an X-ray diffraction diagram of the uranium(IV) phosphateobtained in Example 2;

FIG. 4 is the infrared spectrum of this uranium(IV) phosphate.

DETAILED ACCOUNT OF THE EMBODIMENTS Example 1 Preparation of the ThoriumPhosphate/Diphosphate (TPD)

a) Preparation of the Precursor

A thorium solution is prepared by dissolving 4 g of solid ThCl₄ in a 2Mhydrochloric acid solution in a Teflon container to obtain a thoriumconcentration of 0.7M. A 5M phosphoric acid H₃PO₄ solution, obtained bydiluting approximately 14M concentrated acid with deionized water, isgradually added to this solution with stirring to obtain a phosphoricacid/thorium molar ratio of 3/2 with an excess of phosphoric acid of 2%.Stirring is then halted and the container is closed and heated at atemperature of 150° C. on a sand bath for 21 days. The gel initiallyformed is converted under these conditions to a precipitate. Thereaction which occurs results in the production of a crystalline thoriumphosphate.

The container is cooled and the precipitate is allowed to separate bysettling until a clear supernatant is obtained.

The supernatant is removed using a pipette and is replaced withdeionized water. The combined mixture is suspended by stirring, theprecipitate is then allowed to separate by settling and the operation isrepeated until a supernatant is obtained with a pH in the region of thatof the deionized water. The precipitate is then filtered off on asintered glass funnel under vacuum and is then dried at 120° C.

The X-ray diffraction diagram of the dried product is represented inFIG. 1.

The infrared spectrum of the same product is represented in FIG. 2.

If the diagram of FIG. 1 is compared with the diagram of the thoriumphosphate/diphosphate (TPD) illustrated in FIG. 1 of WO 96/30300 [1], itis noticed that this product is different from the thoriumphosphate/diphosphate; it is a precursor of the TPD.

b) Preparation of the TPD

The precursor obtained above can be converted to thoriumphosphate/diphosphate by subjecting it first of all to a precalcinationat 400° C. for 2 h and then to a calcination at a temperature of 1150°C. for 10 h.

The characteristics of the calcined product clearly correspond to thoseof the thorium phosphate/diphosphate of the document [1].

Example 2 Preparation of a Precursor Product of Uranium(IV) Phosphate

a) Preparation of the Precursor

The same procedure is followed as in Example 1. Thus, a uranium(IV)solution is first of all prepared by dissolution of 4 g of UCl₄ in a 4Mhydrochloric acid HCl solution in order to obtain a uraniumconcentration of 0.7M. This solution is mixed with a 5M phosphoric acidsolution in the phosphoric acid/uranium(IV) molar ratio of 3/2 with anexcess of 2% of phosphoric acid. The mixing is carried out in acontainer which is subsequently closed and which is heated, as inExample 1, at a temperature of 150° C. on a sand bath for 1 week. Underthese conditions, the initial gel is converted by heating to acrystalline powder.

The powder is separated, is washed and is then dried as in Example 1.

The X-ray diffraction diagram of the product obtained is represented inFIG. 3.

The infrared spectrum of this product is represented in FIG. 4.

b) Preparation of the Uranium Phosphate

The product obtained above is subjected to a precalcination and to acalcination as in Example 1 and a polyphase uranium phosphate system isthus obtained.

Products based on protactinium(IV) phosphate, on neptunium(IV) phosphateor on plutonium(IV) phosphate can be prepared in the same way by usingthe same phosphoric acid/actinide(IV) molar ratio and by preparing themixture from an acidic protactinium(IV), neptunium(IV) or plutonium(IV)solution.

Example 3 Preparation of a Solid Solution of Phosphate/Diphosphate ofThorium and of Actinide(IV)

In this case, the preparation is carried out of a solution of thoriumand of actinide(IV) in proportions which make it possible to obtain aphosphate/diphosphate of formula Th_(4-x)M_(x)(PO₄)₄P₂O₇ in which Mrepresents the actinide(IV), with x exhibiting the following values:

-   -   x≦3.75 for Pa(IV)    -   x≦3 for U(IV)    -   x≦2.14 for Np(IV)    -   x≦1.67 for Pu(IV).

The thorium solution obtained in Example 1 is mixed with an actinidesolution, and mixing is carried out with a 5M phosphoric acid solutionas in the preceding examples. The amounts of thorium, of actinide M andof phosphoric acid are such that the phosphoric acid/Th+M molar ratio is3/2 with an excess of phosphoric acid of 2%. This mixture issubsequently subjected to heating in a closed container at a temperatureof 150° C. on a sand bath for 1 week.

The gel initially formed is converted to a powder. The powder isseparated, is washed and is dried as in Example 1. The dried powder isvery homogeneous and is composed of particles with a size of less than 3μm. The corresponding specific surface is close to 10 m²/g, whichconfers a higher reactivity thereon.

This high reactivity of the powder renders it highly advantageous forthe conditioning of the radioactive actinides introduced into thethorium phosphate/diphosphate.

This is because a powder is obtained which has better physicochemicalproperties than those of the powder obtained in the document [1].

The powder is subsequently subjected to a heat treatment at atemperature of 1250° C. for 10 h to form the solid solution ofphosphate/diphosphate of thorium and of actinide(IV) of formulaTh_(4-x)M_(x)(PO₄)₄P₂O₇.

The specific surface of the powder significantly decreases for heattreatment temperatures greater than or equal to 800° C. Correspondingly,the size of the particles increases until it reaches 10 to 20 μm at1250° C.

If, prior to the heat treatment, the powder is compacted at 500 MPa, thedensity of the product reaches 95% of that calculated, after only 5hours of heat treatment, which corresponds to approximately 5% of totalporosity, approximately equally divided between open porosity and closedporosity.

Furthermore, the specific surface of the pellets is between 750 and 1500cm²/g, which makes it possible, during leaching tests, to reduce by afactor of approximately 6 the rate of dissolution of the solid withrespect to that of the powder. In addition to the influence of thespecific surface on the rate of leaching, studies undertaken confirm thevery good physicochemical properties of confinement of the solidsolution of phosphate/diphosphate of thorium and of tetravalentactinide, such as uranium(IV).

Example 4 Preparation of a Composite Sintered Glass Based onPhosphate/Diphosphate of Thorium and of Uranium(IV) and on GadoliniumPhosphate

Solutions of thorium (0.7M) and of uranium(IV) (0.6M) in a hydrochloricacid medium are mixed, as described in the preceding examples, so as toobserve a Th/U molar ratio of 1.5, and then a 5M phosphoric acidsolution is added in the PO₄/(Th+U) molar ratio of 1.5. This mixture isplaced in a closed container and is heated on a sand bath at 150° C. for2 days. The crystalline precipitate of phosphate of thorium and ofuranium(IV) thus obtained is filtered off, washed and then dried asdescribed in Example 3.

Beforehand, a gadolinium phosphate powder is prepared by precipitation(or evaporation to dryness), heated at 150° C., filtered off, dried,milled and then treated at 1250° C. for 10 hours. The GdPO₄ powder thencrystallizes in the monazite structure. It is homogeneous andsingle-phase and is characterized by a specific surface of 1 to 3 m²/gand a mean particle size of less than 2 μm, which confers a highreactivity thereon.

This powder is dispersed in the phosphate of thorium and of uranium(IV)in the phosphate of thorium and of uranium(IV)/gadolinium phosphateratio by mass of 70/30. The mixture is milled, compacted at 500 MPa andthen treated at 1250° C. for 10 hours under an inert atmosphere (forexample, under an argon atmosphere). After heat treatment at 1250° C., adense sintered glass is thus obtained, formed of a solid solution ofphosphate/diphosphate of thorium and of uranium(IV) of formulaTh_(2.4)U_(1.6)(PO₄)₄P₂O₇ comprising, in dispersion, gadoliniumphosphate GdPO₄ with a structure of monazite type.

The gadolinium (used as neutron poison) can be partially substituted byamericium and/or curium, while the uranium(IV) can be substituted byneptunium(IV) and/or plutonium(IV), which then makes it possible toprepare composite samples simultaneously comprising trivalent andtetravalent actinides.

Example 5 Separation of Uranium(VI) from Divalent, Trivalent andTetravalent Ions in Solution

The starting material is a solution comprising 0.1M thorium nitrate and0.1M uranyl nitrate in 2M nitric acid, and a 5M phosphoric acid H₃PO₄solution is added thereto in order to have a phosphoric acid/thoriumstoichiometric ratio of 1.5 with 2% excess of phosphoric acid. A gel isthus formed, which gel includes the ions, and then the gel is heated ina closed container at a temperature of 150° C. until a precipitate isobtained which comprises the precursor of the thoriumphosphate/diphosphate, whereas the uranyl ion remains in thesupernatant.

Uranium(VI) can thus be recovered by separation by settling, filtrationand washing.

If the solution comprises other divalent cations, such as Ca²⁺, Ba²⁺ andSr²⁺, and other trivalent cations, such as La³⁺, Gd³⁺ and Ce³⁺, thelatter are entrained with the thorium phosphate in the precipitate formwhereas uranium(VI) remains in solution.

It is the same if the solution comprises other tetravalent actinides,which will be entrained in the solid phase with the thorium.

The same procedure can be followed for decontaminating radioactiveliquid effluents.

REFERENCE CITED

-   [1] WO 96/30300

1. Process for the preparation of a product comprising a phosphate of at least one element M(IV) chosen from thorium(IV) and actinide(IV)s, wherein it comprises the following stages: a) mixing a solution comprising thorium(IV) and/or at least one actinide(IV) with a phosphoric acid solution in amounts such that the molar ratio $\frac{{PO}_{4}}{M\;({IV})}$  where M(IV) represents the total concentration of thorium(IV) and/or actinide(IV)s, is from 1.4 to 2, b) heating the mixture of the solutions in a closed container at a temperature of 50 to 250° C. to precipitate a product comprising a phosphate of at least one element M chosen from thorium(IV) and actinide(IV)s having a P/M molar ratio of 1.5, and c) separating the precipitated product from the solution.
 2. Process according to claim 1, which additionally comprises the following stages: d) washing with water the precipitated product thus separated, and e) drying the washed product.
 3. Process according to claim 1, in which the thorium solution is a solution of ThCl₄ in hydrochloric acid.
 4. Process according to claim 1, in which the actinide is uranium(IV) and the uranium(IV) solution is a solution of UCl₄ in hydrochloric acid.
 5. Process according to claim 1, in which the actinide is neptunium(IV) and the neptunium(IV) solution is a solution of neptunium in nitric acid.
 6. Process according to claim 1, in which the actinide is plutonium(IV) and the plutonium(IV) solution is a solution of plutonium in nitric acid.
 7. Process according to claim 1, in which, in stage a), at least one element chosen from trivalent actinides and trivalent lanthanides is additionally added to the mixture in order to include trivalent actinide(s) and/or trivalent lanthanide(s) in the product precipitated in stage b).
 8. Process for the preparation of thorium phosphate/diphosphate of formula Th₄(PO₄)₄P₂O₇, wherein a product based on thorium(IV) phosphate is prepared by carrying out the process according to claim 1 using, in stage a), a solution comprising thorium and phosphoric acid and wherein the product based on thorium(IV) phosphate is subjected to a heat treatment carried out at least partially at a temperature of 700° C. to 1300° C.
 9. Process according to claim 8, in which the heat treatment is carried out in two stages which are respectively a first stage carried out at a temperature of 300 to 500° C. for 1 h to 5 hours and a second stage carried out at a temperature of 1100 to 1300° C. for 3 to 15 h.
 10. Process for the preparation of uranium phosphate, wherein a product based on uranium(IV) phosphate is prepared by carrying out the process according to claim 1 using, in stage a), a solution comprising uranium and phosphoric acid and wherein the product based on uranium(IV) phosphate is subjected to a heat treatment carried out at least partially at a temperature of 700° C. to 1300° C.
 11. Process according to claim 10, in which the heat treatment is carried out in two stages which are respectively a first stage carried out at a temperature of 300 to 500° C. for 1 h to 5 hours and a second stage carried out at a temperature of 1100 to 1300° C. for 3 to 15 h.
 12. Process for the preparation of a solid solution of phosphates of thorium and of at least one tetravalent actinide, wherein a product comprising phosphates of Th and of at least one tetravalent actinide is prepared by carrying out the process according to claim 1 using, in stage a), a solution comprising thorium and at least one actinide and phosphoric acid and wherein the product based on phosphates of thorium(IV) and of actinide(IV) is subjected to a heat treatment carried out at least partially at a temperature of 700° C. to 1300° C.
 13. Process according to claim 12, in which the solid solution of phosphates corresponds to the formula: Th_(4-x)M_(x)(PO₄)₄P₂O₇ in which M is an element chosen from Pa(IV), U(IV), Np(IV) and Pu(IV), and x satisfies the following conditions: x≦3.75 for Pa(IV) x≦3 for U(IV) x≦2.14 for Np(IV) x≦1.67 for Pu(IV).
 14. Process according to claim 12, in which the heat treatment is carried out in two stages which are respectively a first stage carried out at a temperature of 300 to 500° C. for 1 h to 5 hours and a second stage carried out at a temperature of 1100 to 1300° C. for 3 to 15 h.
 15. Process for the preparation of a composite material including at least one actinide(III) and/or at least one lanthanide(III), wherein the precipitate comprising a phosphate of Th(IV) and/or of actinide(IV)(s) is prepared by carrying out the process according to claim 1 and wherein a powder comprising at least one actinide(III) and/or at least one lanthanide(III) in the phosphate form is dispersed in the precipitate and wherein the combination is subsequently subjected to a heat treatment, optionally preceded by a compacting, carried out at least partially at a temperature of 700 to 1300° C.
 16. Process for the separation of uranium(VI), in the form of the uranyl ion UO₂ ²⁺, present in a solution with other cations, including thorium, wherein phosphoric acid is added to the solution in an amount such that the molar ratio $\frac{{phosphoric}\mspace{20mu}{acid}}{{other}\mspace{20mu}{cations}}$ is from 1.4 to 2, wherein the solution thus obtained is heated at a temperature of 50 to 250° C. in a closed container, in order to precipitate a product comprising the cations other than uranium, and wherein the solution comprising uranium(VI) is recovered.
 17. Process for the decontamination of radioactive aqueous effluent comprising contaminating radioactive cations, the process comprising: adding thorium and then phosphoric acid to the effluent in amounts such that the P/Th molar ratio is from 1.4 to 2; heating the effluent in a closed container at a temperature of 50 to 250° C.; and precipitating a product from the effluent comprising thorium phosphate and entrained contaminating radioactive cations. 